Fifty hyperactive children new to stimulant medication and 50 age-matched normal children will be selected by means of standardized teachers; and parents' ratings. Hyperactive patients will participate in three identical sessions prior to which they will receive placebo, a moderate (0.3 mg/Kg). or a high dose of methylphenidate (0.6 mg/Kg) in random order and multiple-blind fashions. Normal children will be tested without medications. Subjects' evoked potentials will be recorded under photic stimulation and three vigilance tasks. All these procedures have yielded significant drug effects or diagnostic differences in our own or others' laboratories. The aim of this research is to pursue these differences as a function of different dosage levels of medication.